Current fastener manufacturing encompasses six core processes, each designed with specific materials, performance requirements, and cost considerations in mind:
Cold Heading Process
Cold heading is the most prevalent forming method in fastener production. It involves using high-pressure dies to extrude metal wire (such as carbon steel, stainless steel, or aluminum alloy) at room temperature into basic shapes like bolts and nuts. The core advantages of this process are high efficiency and superior material utilization; a single forming operation can reduce the need for subsequent machining (cutting) by 30%–50%, while achieving dimensional precision within ±0.05mm. Typical applications include automotive engine bolts and high-strength construction nuts-components that demand exceptional strength and consistency. Key technical aspects include:
- Die Design: Optimizing die cavity angles based on material ductility to prevent cracking;
- Lubrication: Using graphite or water-based lubricants to reduce the coefficient of friction and extend die life;
- Annealing: Performing spheroidizing annealing after cold heading to relieve internal stress and improve workability for subsequent processing.
Hot Forging Process
For large-scale or high-strength fasteners (such as wind turbine tower bolts and bridge connectors), the hot forging process involves heating the metal above its recrystallization temperature (typically 800–1200°C) and applying pressure via a forging press to induce plastic deformation. Key advantages include the ability to produce complex cross-sectional shapes and achieve high material density, resulting in a tensile strength 15%–20% higher than that of cold-headed parts. Critical control points include:
- Temperature control: Strict monitoring of heating temperature and holding time is required to prevent excessive oxide scale formation or grain coarsening;
- Forging ratio: A total deformation ratio of ≥3 is typically required to ensure continuous metal grain flow;
- Post-processing: Hardness adjustment via normalizing or quenching and tempering is necessary to meet the standard HRC 28–38 range.
Turning Process
Although less efficient than cold heading, the turning process remains indispensable for producing specially shaped fasteners (such as non-standard bolts and drilled pins). The process involves rotating the workpiece on a CNC lathe and using a cutting tool to remove excess material. Key technical features include:
- High precision: Achieves IT5–IT7 tolerance grades, making it suitable for precision instrument connectors;
- Broad material adaptability: Capable of processing materials that are difficult to form via cold heading, such as copper and titanium alloys;
Stamping Process
Primarily used to manufacture flat fasteners such as washers and spring washers, this process employs a power press to drive a die that performs operations like blanking and bending on metal sheet stock. Its key advantages are high-speed production (capable of hundreds of parts per minute) and low cost; however, the following factors require attention:
- Die Wear: Punches and dies must be replaced periodically to prevent excessive burrs;
- Material Selection: Low-carbon steel sheets with good ductility, such as Q235 or SPCC, are preferred;
- Surface Treatment: Post-stamping treatments-such as electroplating or blackening-are necessary to prevent rust and corrosion.
Thread Rolling Process
Used for threading bolts and screws, the thread rolling process forms threads on the workpiece surface through the rotational compression of two or three rolling dies. Compared to thread cutting, its advantages include:
- Increased Strength: Cold work hardening boosts the shear strength of the threads by 20%–30%;
- Surface Quality: Thread surface roughness can reach Ra 0.8 μm, reducing frictional loss;
- Material Savings: No chips are produced, resulting in material utilization approaching 100%.
Surface Treatment Processes
The corrosion resistance and aesthetic quality of fasteners depend on surface treatment; common processes include:
- Electro-galvanizing: Low cost, suitable for indoor environments, with a typical thickness of 5–12 μm;
- Dacromet: No risk of hydrogen embrittlement, withstands 500–1,000 hours of salt spray testing, and is commonly used for automotive chassis components;
- Zinc diffusion coating: Forms a zinc-iron alloy layer on the steel surface via diffusion, offering 3–5 times the corrosion resistance of electro-galvanizing;
- Spraying: Such as polyester powder coating, used for architectural decorative screws to provide a wide range of color options.